1. Field of the Invention
The present invention relates to a crystallization apparatus and a crystallization method. More particularly, the present invention relates to a crystallization apparatus and a crystallization method for forming a crystallized semiconductor film by irradiating a polycrystal semiconductor film or an amorphous semiconductor film with laser beams having a predetermined light intensity distribution.
2. Description of the Related Art
A thin film transistor (TFT) which is used in a switching element or the like which controls a voltage to be applied to pixels of, e.g., a liquid crystal display (LCD) is formed to an amorphous silicon or polysilicon layer.
A polysilicon layer has a higher electron mobility than that of amorphous silicon. Therefore, when a transistor is formed to the polysilicon layer, a switching speed and a display response are faster than those in cases where the transistor is formed to the amorphous silicon. Further, a peripheral LSI can be constituted of a thin film transistor. Furthermore, there is an advantage that a design margin of any other component can be decreased. Moreover, peripheral circuits such as a driver circuit or a DAC can be operated at higher speeds when they are incorporated in a display.
Although the polysilicon is formed of an aggregation of one-crystal grains, its electron mobility is lower than that of single-crystal silicon. Additionally, a thin film transistor formed to the polycrystal silicon has a problem of irregularities in crystal grain boundary number in a channel portion.
Thus, in recent years, in order to improve the electron mobility and reduce irregularities in crystal grain boundary number in the channel portion, there has been proposed a crystallization method which generates crystallized silicon with a large particle size.
As this type of crystallization method, there is known a “Phase Control Excimer Laser Annealing (ELA method)” which generates a crystallized semiconductor film by irradiating a phase shifter which is caused to be in contiguity with a polycrystal semiconductor film or an amorphous semiconductor film in parallel with excimer laser beams. The details of the phase control ELA are disclosed in, e.g., “Surface Science Vol. 21, No. 5, pp. 278-287, 2000”.
In the phase control ELA, a light intensity distribution with an inverse peak pattern that a light intensity is substantially zero at a point corresponding to a phase shift portion of a phase shifter is generated, and a polycrystal semiconductor film or an amorphous semiconductor film is irradiated with the light beams having this light intensity distribution with the inverse peak pattern. It is to be noted that the inverse peak pattern is defined as a pattern that a light intensity is substantially zero at the center and the light intensity is suddenly increased toward the periphery. As a result, a temperature gradient is generated in a fusion area in accordance with the light intensity distribution, and a crystal nucleus is formed at a part which is solidified first in accordance with the point where the light intensity is substantially zero. Therefore, a crystal grows in a lateral direction from that crystal nucleus toward the periphery (which will be referred to as a “lateral growth” hereinafter), thereby generating single-crystal grains with a large particle size.
Meanwhile, a phase shifter which is generally used in the prior art is a line type phase shifter, and comprises two rectangular areas which are alternately repeated along one direction. A phase difference of π (180 degrees) is given between the two alternately repeated areas.
FIGS. 11A to 11C are views illustrating a structure and an effect of a line type phase shifter. When the line type phase shifter is used, as shown in FIG. 11A, a linear border line 101c between two areas 101a and 101b having a phase difference of, e.g., 180 degrees constitutes a phase shift line. Therefore, as shown in FIG. 11B, there is formed a light intensity distribution with an inverse peak pattern that a light intensity is substantially zero on a line 102 corresponding to the phase shift line and the light intensity is one-dimensionally increased toward the periphery in a direction orthogonal to the line 102. In this case, as shown in FIG. 11C, a temperature distribution becomes lowest along the line 102 corresponding to the phase shift line, and a temperature gradient (indicated by arrows) is generated along the direction orthogonal to the line 102 corresponding to the phase shift line. As a result, a crystal nucleus is generated in the vicinity of the line 102 corresponding to the phase shift line, and crystallization advances from that crystal nucleus along the line orthogonal to the line 102 corresponding to the phase shift line.
FIGS. 12A to 12C are views illustrating inconveniences when using light beams to which a light intensity distribution with an inverse peak pattern obtained by a use of the phase shifter depicted in FIG. 11A is given. As shown in FIG. 12A, in an area where the light intensity is not more than a reference intensity A, amorphous silicon does not change and remains in an amorphous silicon state, or it remains in a polysilicon state even if it is fused. As a result, the crystal growth does not start (a state including this amorphous state and the polysilicon state will be referred to as a “non-crystallized area” hereinafter for convenience's sake). It can be considered that a peripheral portion of this non-crystallized area becomes a crystal nucleus and a crystal grows from there. Therefore, the first importance is the light intensity in an inverse peak value (point where the minimum light intensity is minimum), i.e., a value α when the maximum value of the light intensity in the light intensity distribution with the inverse peak pattern is standardized as 1. Incidentally, as shown in FIG. 12B, when the value α exceeds the reference intensity A and becomes too large, the temperature gradient becomes small, and hence there is a problem that the crystal growth tends to stop halfway.
Generally, in the prior art using, e.g., the line type phase shifter, as shown in FIG. 12C, the value α becomes too small as compared with the first reference intensity A and the non-crystallized area tends to become large. Considering that the phase shift line (shown in FIG. 11C) is arranged with a fixed pitch, it is hard to obtain a crystal with a large particle size. Additionally, as shown in FIG. 12A, in an area where the light intensity is not less than a second reference intensity B, an amorphous silicon semiconductor is lost or destructed due to ablation (transpiration). Since the value α is adjusted based on a shape of the phase shifter or optical conditions in the prior art in this manner, adjusting the value α also changes properties of the light intensity distribution. Therefore, it is difficult to obtain a desired light intensity distribution in order to realize the sufficient lateral growth from a crystal nucleus.